It's sad that your daughter has put you in the middle of her issues. Given that your children are adults, your best bet is to stay completely out of it.
I wouldn't recommend contacting her about the lump, at least until you know something. Lumps can be very scary but they aren't always cancer. She may perceive any contact about it as being an attempt to draw her back in, and if it turns out to be benign she may be suspicious of whether there was anything in the first place. (NOT saying that you're making it up, not at all, just saying that may be how she perceives it.)
I wish you all the best. The not knowing can often be worse than knowing.
I'm sick to death of it - anyone else?
What on earth do I do? - too close to Christmas
I don't understand this trend - AIBU?